Lena Dunham’s recent revelation of sexual activity with her sister, who is six years her junior, has spurred discussion on the Internet about whether what she did constitutes abuse. There seems to be a lot of confusion about where the line is, even for skilled therapists.

As a psychologist who at one time trained teachers, parents, and children about abuse while working for the Los Angeles Unified School District, the rule is that if there is a big age discrepancy between siblings, then what looks like curiosity can be abusive.

When a big sister or brother looks at their infant sibling’s genitals, or even touches them, that is normal curiosity. When a parent does not set appropriate boundaries and such touching or viewing continues, that may be abuse. When such activity continues on in secret, then there is a question of adequate supervision by the parent, especially if the parent already knows that there has been inappropriate activity.

Here is the thing: You cannot talk about childhood sexuality in the same way that you talk about adult sexuality. Normal children are curious about each other’s bodies. Normal children do touch one another because it feels good. When they are similar in age, looking and touching are expected and considered to be fine from a developmental standpoint. When there is a big difference in age (or even size) that often indicates there is a problem.

A younger or smaller sibling cannot participate in consensual exploration, because they cannot understand what it is they are consenting to. They do not have the language to describe what they are experiencing, nor the context of certain kinds of touch. Thus it is inappropriate for the older child to explore the other child’s intimate body parts.

A one-time event is an opportunity for a parent to explain, in language a child can understand, that a younger sibling may not understand what is happening to their body, even if the older child intends to be playful or even affectionate. It may also an opportunity to answer a child’s questions about bodies and sex, since they are showing curiosity.

To explain boundaries, an analogy can be made to a family pet. A child might think it is funny to wrap string around an animal’s jaw and watch them wriggle, but it must be explained that the animal may not find it funny, even though the animal has no way of saying this. It can be emphasized that the pet depends on others to be nice to it, since it cannot communicate. In the same way, younger siblings depend on older siblings to be nice to them.

Sibling abuse is much more common than most people think. In my sex therapy practice, many people confide in me that a sibling sexually abused them. Sometimes they even want to know if what happened was “really” sexual abuse. If the sibling coerced them or forced them in some way, such as holding them down while performing a sexual act, or if the sibling made threats (“I’ll tell mom and dad that you did this to me!”), then yes, it is abuse.

Sexual abuse between siblings is sometimes—though not always—indicative that there may be other types of inappropriate sexual behavior going on in a child’s life. Children don’t learn certain kinds of sex acts out of the blue. It is important to at least investigate the possibility when an older sibling is found performing sexual acts on a younger sibling.

Putting a spotlight on sibling-on-sibling sexual abuse may create discomfort for millions of people. If you are an adult who was sexually abused by a sibling and feel affected by it still, get help. There are psychotherapists who have specialized training to help you understand how to cope with the effects of abuse and even how to manage current family relationships. You don’t have to go on alone with your painful feelings.